Ion radicals

A radical ion is a free radical species that carries a negative charge (radical anion) or a positive charge (radical cation). When a neutral, spin-paired species gains a single electron it becomes a radical anion. Likewise, when a neutral, spin-paired species loses an electron it becomes a radical cation. 

Radical cations and radical anions are known in the gas phase. They are routinely generated and studied in the complementary techniques of mass spectrometry and negative ion mass spectrometry. 

Many aromatic compounds can undergo one-electron reduction by alkali metals, such as Na and Li. For example, the reaction of naphthalene with sodium in an aprotic solvent gives the naphthalene radical anion - sodium ion salt. 

Cationic radicals are much less stable and noticed prominently in mass spectroscopy. When a molecule in gas phase is subjected to electron ionization, one electron is abstracted by the electron beam to create a radical cation. This species represents the molecular ion or parent ion, which on fragmentation gives a complex mixture of ions and uncharged radical species. For example, the methanol radical cation fragments into a methyl cation CFl and a hydroxyl radical. Secondary species are also generated by proton gain (M -F 1) and proton loss (M — 1). 

Radical ions are, in the main, not very important as active centres of polymerizations. In media suitable for the existence both of radicals and of ions, the latter are usually more reactive. Moreover, the radicals decay by combination their contribution to chain propagation is usually negligible. Radical ions are more important as precursors of active centres, as intermediates generated from initiators and monomers through their radical ends they can combine (disproportionate) yielding active centres, frequently diions. Studies of radical ion behaviour contribute to our knowledge of the processes connected with electron transfer from molecule to molecule. These oxidation-reduction processes are very important in macromolecular chemistry. 

Our knowledge of the energy states of a biatomic molecule capturing an electron in the gas phase is sufficiently precise [167], The electron affinities 

A solvated electron may be regarded as the simplest radical anion. The chemistry of solvated electron reactions is qualitatively similar to radical anion reactions but the physical properties of electron solutions are very complicated [167]. They are not suitable as a model of radical anions. 

For some types of anionic polymerization, the equilibrium between the alkali metal and the aromatic hydrocarbon is important 

The heat of this reaction and the change in the Gibbs energy (for gas ions, solid metal D and crystalline acceptor A) are given by the relations [167c] 

The acetylene anion radical undergoes autodetachment of the electron, but the vinylidene anion can be generated easily [83]. Since the calculated isomerization barrier is -45 kcal/mol, the 2B2 ground-state vinylidene anion radical is predicted to be stable with respect to the 1,2-hydrogen shift [30, 84, 85]. As mentioned before, the vinylidene anion radical was used as the precursor for the generation of the singlet vinylidene in Lineberger s experimental studies. 

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Miller T A 1982 Light and radical ions Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and FI L Strauss (Palo Alto, CA Annual Reviews)... 

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

Gould I R, Ege D, Mattes S L and Farid S 1987 Return electron transfer with geminate radical ion pairs - observation of the Marcus inverted region J. Am. Chem. Soc. 109 3794-6... 

In aniline derivatives (458) the mechanism of this reaction is still not fully settled (459-461). However, the latest results seem to favor a pathway that, applied to 2-nitraminothiazole, would give Scheme 138, where the key step is the formation of a radical ion (223). Reexamination of the original reports on this reaction (16, 374, 378. 462) with EPR and Chemically Induced Dynamic Nuclear Polarisation techniques could be fruitful. 

Free Radicals. In the formula of a polyatomic radical an unpaired electron(s) is(are) indicated by a dot placed as a right superscript to the parentheses (or square bracket for coordination compounds). In radical ions the dot precedes the charge. In structural formulas, the dot may be placed to indicate the location of the unpaired electron(s). 

Radical ion. An ion containing an unpaired electron that is thus both an ion and a free radical. The presence of the odd electron is denoted by placing a dot alongside the symbol for the charge. Thus, and SF are radical ions. 

The catalyst is prepared by the reaction of sodium metal with naphthalen( and results in the formation of a radical ion ... 

These green radical ions react with styrene to produce the red styrem radical anions ... 

Electron-transfer reactions producing triplet excited states can be diagnosed by a substantial increase in luminescence intensity produced by a magnetic field (170). The intensity increases because the magnetic field reduces quenching of the triplet by radical ions (157). 

W. H. PoweU, RepisedNomenclaturefor Radicals, Ions, Radicallons, and Related Species, lUPAC Recommendations, 199 r,PureAppl Chem. 65, 1357 (1993). 

Aqueous Phase. In pure water, the decomposition of ozone at 20°C iavolves a complex radical chain mechanism, initiated by OH and propagated by O2 radical ions and HO radicals (25). O3 is a radical ion. 

Hydrogen peroxide greatly accelerates the decomposition of ozone in alkaline solutions because of formation of HOg, which reacts rapidly with ozone to form the radical ion (25). When the concentration of H2O2 exceeds 10 Af, the decomposition of ozone is initiated faster by HOg than by... 

Inorganic Reactions. Ozone reacts rapidly with various free radicals and radical ions such as O, 0 , H, HO, N, NO, Cl, and Br. Some of these radicals (HO, NO, Cl, and Br) can initiate the catalytic decomposition of ozone. 

The hterature suggests that more than one mechanism may be operative for a given antiozonant, and that different mechanisms may be appHcable to different types of antiozonants. All of the evidence, however, indicates that the scavenger mechanism is the most important. All antiozonants react with ozone at a much higher rate than does the mbber which they protect. The extremely high reactivity with ozone of/)-phenylenediamines, compared to other amines, is best explained by their unique abiUty to react ftee-tadicaHy. The chemistry of ozone—/)-PDA reactions is known in some detail (30,31). The first step is beheved to be the formation of an ozone—/)-PDA adduct (32), or in some cases a radical ion. Pour competing fates for dissociation of the initial adduct have been described amine oxide formation, side-chain oxidation, nitroxide radical formation, and amino radical formation. 

The added electron is delocalized on the monovalent radical ion to which it is reduced (3). There is no general agreement on the molecular representation of the reduced stmcture. Various other viologen compounds have been mentioned (9,12). Even a polymeric electrochromic device (15) has been made, though the penalty for polymerization is a loss in device speed. Methylviologen dichloride [1910-42-5] was dissolved in hydrated... 

At 70—140°C, peroxide is vaporised. Peroxide vapor has been reported to rapidly inactivate pathogenic bacteria, yeast, and bacterial spores in very low concentrations (133). Experiments using peroxide vapor for space decontamination of rooms and biologic safety cabinets hold promise (134). The use of peroxide vapor and a plasma generated by radio frequency energy releasing free radicals, ions, excited atoms, and excited molecules in a sterilising chamber has been patented (135). 

Sodium hydrosulfite or sodium dithionate, Na2S204, under alkaline conditions are powerful reducing agents the oxidation potential is +1.12 V. The reduction of -phenylazobenzenesulfonic acid with sodium hydrosulfite in alkaline solutions is first order with respect to -phenylazobenzenesulfonate ion concentration and one-half order with respect to dithionate ion concentration (135). The SO 2 radical ion is a reaction intermediate for the reduction mechanisms. The reaction equation for this reduction is... 

Information regarding the position of the substituents can be obtained from the mass spectra of the enamines of cyclic ketones. For instance in the case of the morpholine enamine of 3-methylcyclohexanone, which is shown to be a 2 1 mixture of/ and isomers by NMR spectroscopy, the fragmentation of the radical ion from the /) isomer results in the loss of a methyl radical from the C-3 position. The d isomer gives a complicated spectrum due to the loss of the hydrogen radical. 

Another interesting example is found in the morpholine enamine of 2- -propylcyclohexanone (138), which consists of a 2 3 mixture of tri- and tetrasubstituted isomers. The radical ion from the tetrasubstituted isomer loses an ethyl radical, giving the base peak at m/e 180. 

The dithionite ion has a remarkable eclipsed stmcture of approximate C2v symmetry (Fig. 15.32b). The extraordinarily long S-S distance (239 pm) and the almost parallel SO2 planes (dihedral angle 30°) are other unusual features. Electron-spin-resonance studies have shown the presence of the S02 radical ion in solution ( 300ppm), suggesting the establishment of a monomer-dimer equilibrium 8204 ... 

This interaction dominates the spectra for free radicals and radical ions in solids. It averages to zero for species in the gas phase or in solution. 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

This aminium radical salt in aqueous solution in the form of solvated radical salt is very stable and will not polymerize acrylonitrile even with CeHsCOONa to form the corresponding benzoate. Therefore, we believe that in the nucleophilic displacement, there must be some intermediate step, such as intimate ion pair and cyclic transition state, which will then proceed the deprotonation to form the active aminium radical ion [14], as shown in Scheme 1. The presence of the above aminomethyl radical has also been verified [15] through ultraviolet (UV) analysis of this polymer formed such as PAN or PMMA with the characteristic band as the end group. 

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